Digital communication receivers must sample an analog waveform and then reliably detect the sampled data. Signals arriving at a receiver are typically corrupted by intersymbol interference (ISI), crosstalk, echo and other noise. Generally, intersymbol interference is caused by variations in group delay through the connection media, which is a function of the transmitted data pattern. This causes the data eye to be closed in the horizontal direction (timing wise) and vertical direction (amplitude attenuation of the serial data at the CDR input).
In order to mitigate these impairments, typical communication receivers contain arrangements for linear equalization or decision feedback equalization (or both). Linear equalization is a feed forward equalization that typically provides for amplification and high pass filtering of the incoming serial data. Decision feedback equalization (DFE) is a widely-used technique for removing intersymbol interference and other correlated noise. For a detailed discussion of decision feedback equalizers, see, for example, R. Gitlin et al., Digital Communication Principles, (Plenum Press 1992) and E. A. Lee and D. G. Messerschmitt, Digital Communications, (Kluwer Academic Press, 1988), each incorporated by reference herein. Generally, decision-feedback equalization utilizes a nonlinear equalizer to equalize the channel using a feedback loop based on previously received symbols.
U.S. patent application Ser. No. 12/600,749, to Philip Jenkins et al. and entitled “Continuous Time-Decision Feedback Equalizer,” incorporated by reference herein, discloses a Continuous Time-Decision Feedback Equalizer (CT-DFE) that provides for serial data correction based on the history of the previously received data. In this manner, the disclosed CT-DFE compensates for intersymbol interference.
Generally, CT-DFEs aim to remove post-cursor ISI by using proper pole and gain values to compensate for the channel ISI. Existing CT-DFE adaptation methods use one or more predefined fixed patterns for the pole and gain adaptation. These predefined patterns cannot be changed to a pattern outside of the predefined set. Such fixed adaptation patterns, however, may lead to inferior adaptation results in some applications and in particular adaptation ranges. For example, the contribution from different samples through the CT-DFE feedback path to the error (transition sample as discussed below) may change sign. In addition, at the time of adaptation, some of the patterns may not be available in the incoming data or their frequency of occurrences may be too low.
A need therefore exists for improved CT-DFE adaptation methods and apparatus that employ programmable adaptation patterns for gain and pole settings.